1. Field of the Invention
The present invention relates to a liquid-crystal display and its drive circuit and, more particularly, to a liquid-crystal display and a drive circuit suitable for a polycrystalline silicon liquid-crystal display (LCD) in which drive circuits are integrated over a wide area on a glass substrate.
2. Description of the Related Art
The present invention relates to a drive circuit for applying a voltage to liquid-crystal cells or to a data driver (which may be referred to as an address driver). Herein, a liquid-crystal display employing thin-film transistors (TFTs) will be used as an example. However, the present invention is not limited to this type of liquid-crystal display but can be applied to a data driver in any other type of liquid-crystal display.
According to a prior art, in a liquid-crystal display, cell electrodes and bus electrodes are formed on a glass substrate. A drive circuit is formed on an IC chip, and the IC chip is affixed to the glass substrate. Electrode pads on the IC chip and electrode pads on the glass substrate are linked by bonding wires. In this case, since a drive circuit is formed on a monocrystalline silicon substrate, complex circuitry can be designed. The liquid-crystal display of the prior art therefore includes two-stage of sample-and-hold circuits. While the first stages of sample-and-hold circuits are sampling display data, the second stages of sample-and-hold circuits provide outputs to be written. Data can therefore be written over nearly the whole display data hold period, and a driver of low driving ability can be used.
However, when an IC chip is affixed to a glass substrate and bonding wires are used for making electrical connections, it is difficult to attain sufficient productivity. This poses the problem of an increase in cost. For avoiding this kind of problem, a liquid-crystal display (LCD) using no IC chip such as a poly-crystalline silicon LCD in which a poly-crystalline silicon is used to form circuits directly in a wide area on a glass substrate has been devised. However, the poly-crystalline silicon LCD has a problem that it is more difficult to increase the density of devices to be integrated and stabilize the characteristics of the devices than it is when an IC chip is used. A data driver referred to as a point-sequential driving type data driver, using no sample-and-hold circuits, is employed.
A circuit employing a source-follower amplifier is widely used as a driver because of its simple circuitry. A buffer using a source-follower amplifier outputs a voltage produced by subtracting the threshold voltage of a transistor from an input voltage through the source-follower amplifier. However, there is a problem that the output voltage of the buffer is susceptible to the variation in the characteristic of a device, and changes along with the variation the characteristic. The same applies to a buffer using an operational amplifier. When a gradation level (voltage) must be written, there arises a problem that since an output voltage changes due to the variation in the characteristic of a device, irregular display occurs and display quality deteriorates.
Moreover, the buffer using an operational amplifier has another problem in that the circuitry is so complex as to increase the size of a driver.
The two problems are common to drive circuits for liquid-crystal displays. In particularly, for the poly-crystalline silicon LCD in which drive circuits are integrated over a wide area on a glass substrate, the problems are so critical as to dominate the possibility of attaining a larger screen or higher definition. There is therefore an increasing demand for a compact buffer not susceptible to the characteristics of a device and having simple circuitry.
Moreover, the point-sequential driving type data driver has the merits of having very simple circuitry and of minimizing a decrease in yield, and is therefore widely used for a poly-crystalline silicon LCD, used as a compact panel of up to 2 inches wide, for a projector. However, according to point-sequential driving method, a time interval usable for one writing is as short as several tens to several hundreds of nanoseconds. The data driver must apply a voltage representing data to a data bus for the short time interval. When the data driver is employed in a compact panel for a projector or the like which is conformable to the Video Graphics Array (VGA) standard (640 by 480 pixels) stipulating a relatively low resolution, the capacitance of the data bus is very small and a time constant dependent on the capacitance and resistance of the data bus is very small. Therefore, display data Vdata can be input in a parallel form and the size of an analog switch can be optimized. However, when the data driver is employed in a direct-vision panel to be mounted in a notebook-sized personal computer and having a width of 10 inches or more, the area of a data bus is large. Therefore, the capacitance of the data bus is large, and the time constant is large. Consequently, the point-sequential driving type data driver fails to drive a data bus. Even if the data driver can drive a data bus, there arises a problem that the circuitry required will become unfeasibly large. When an attempt is made to realize a higher resolution, the crossings of wirings will increase. This poses a problem that the point-sequential driving fails to cope with the complex wirings.
As mentioned above, the point-sequential driving type data driver has its limitations in terms of a screen size and resolution. However, as far as the poly-crystalline silicon LCD is concerned, another driving method is unavailable. The foregoing problems are therefore critical problems dominating the possibility of realizing a larger screen or higher definition. There is therefore an increasing demand for a driving method making it possible to ensure a long writing time during which a voltage representing data is applied to a data bus despite small circuitry, and a drive circuit in which the driving method is implemented.